These teachings relate generally to applying high pumping power to a fiber amplifier.
A popular fiber amplifier structure, shown in FIG. 1, comprises a single mode core, doped with an active lasing material, surrounded by a larger multimode cladding of lower refractive index to contain and direct pump radiation into the active core, and itself surrounded with a lower index cladding to prevent radiation loss from the combination.
The fiber amplifier structure can be pumped with input radiation passing into one or both ends of the multimode cladding surrounding the active single mode core in a standard manner.
But pump radiation can be introduced into the multimode pump cladding more effectively and without blocking laser radiation emitted by the single mode core by use of a plurality of tapered multimode fibers fused to the pump cladding, as shown in FIG. 2.
A conventional optical pumping scheme includes optical pumping of a high-power fiber amplifier or laser using a 19:1 tapered fused coupler where each input fiber receives its power from an individual 100 W fiber-coupled module. This conventional optical pumping scheme has the disadvantages of greater complexity, size and weight than are needed with the present invention.
Another conventional exemplary scheme includes optical pumping of a high-power fiber laser using a (6+1):1 tapered fused coupler with two laser bar stacks. The beam from each stack is split into three parts, and each part is then coupled into an optical fiber for pumping. This conventional pumping scheme has greater size and weight, is less mechanically stable than is desirable, is degraded by scattering of light at the lens that splits the beam, and is subject to heat distortion of that lens.
An additional conventional use of a tapered fused coupler for pumping a fiber laser also is larger and heavier than is desirable, is limited in input power by thermal problems at the splitter, loses efficiency at the splitter, and is degraded with unwanted power transmitted into the cladding of its output fibers.
Many types of lasers have been considered as a source of radiant power for pumping a fiber amplifier, including gas, dye, solid rod, and diode lasers. Of these options, the diode laser offers substantially the best cost and portability for high power
Diode lasers produce output power most efficiently when constructed with an emitting region approximately a micron thick and formed with an output width of about 100 microns. For efficient manufacture several such lasers can be placed along a single semiconductor surface, forming a line source (laser diode bar) with multiple emitting facets. Several such laser diode bars can be stacked together vertically in the form of a stack having dozens or hundreds of emitting facets in a two dimensional array, as shown in FIG. 3.
There is a need for systems and methods for accepting the emitted radiant power from such a simple, single combined diode laser structure (laser bar stack) and efficiently directing essentially all of it into the pump cladding of a double-clad fiber amplifier.